1. Field of the Invention
The present invention relates to a load driving circuit with charge spike protection, and more particularly relates to a load driving circuit using a capacitance to function the charge spike protection.
2. Description of Related Art
FIG. 1 is a schematic diagram of a conventional LED driving circuit. The LED driving circuit comprises a converting controller 100, a boost converting circuit 160, an output capacitance C, an LED module 150, a lowest voltage selecting circuit 140, and a current balance circuit 145. The boost converting circuit 160 is a direct current to direct current boost converting circuit, comprising an inductance L, a switch SW and a rectifier device D. One end of the inductance L is coupled to an input power source Vin and the other end thereof is coupled to one end of the switch SW. Another end of the switch SW is grounded. A positive end of the rectifier device D is coupled to the connecting node of the inductance L and the switch SW and a negative end thereof is coupled to the output capacitance C. The output capacitance C receives the power transmitted by the boost converting circuit 160 to generate a driving power source Vout to drive the LED module 150 lighting.
The current balance circuit 145 is coupled to every negative end of the LED string in the LED module 150 to balance the currents of the LED strings. Therefore, every LED string lights equally. The lowest voltage selecting circuit 140 is coupled to every negative end of the LED string in the LED module 150 to determine whose voltage is the lowest and accordingly generates a voltage feedback signal VFB. The converting controller 100 generates a switching signal Sc according to the voltage feedback signal VFB to control the switch SW.
The converting controller 100 is packaged in a package structure comprising a drive pin and a ground pin. Therefore, an electrostatic discharge protection circuit is equipped in the converting controller 100 to transmit the electrostatic energy out by the two pins to avoid the converting controller 100 damaged. The current balance circuit 145 has no drive pin (VDD pin) and so it does not have the electrostatic discharge protection circuit for the VDD pin. Therefore, while the electrostatic energy is transmitted into the current balance circuit 145 by the connecting node of the current balance circuit 145 and the LED module 150, the current balance circuit 145 is easily to be damage due to the electrostatic energy.
FIG. 2 is a schematic diagram of a conventional LED driving circuit with electrostatic discharge protection circuit. Compared with FIG. 1, the circuit in FIG. 2 additionally increases a TVS electrostatic discharge protection circuit 155 which is coupled to the ground and the connecting node of the current balance circuit 145 and the LED module 150. The TVS electrostatic discharge protection circuit 155 is composed by the transient voltage suppressor (TVS). A feature of the TVS is to respond faster and so the TVB protects electronic devises from a transient energy before the electronic devises are damaged. Another feature thereof is lower breakdown voltage and so the TVB is applicable to a low-voltage application. When the electrostatic discharge is produced, no matter it is a positive voltage or negative voltage, it can be transmitted to the ground by the TVS in the TVS electrostatic discharge protection circuit 155 to function the electrostatic discharge protection.
At the same time, a cost of the TVB is high, and total cost of system is substantially increased. Moreover, a tolerance of breakdown voltage of the TVB is large. It results that users may use the TVB with a breakdown voltage much higher than a driving voltage to avoid the TVB conducting in a normal voltage range of the driving voltage. Therefore, the breakdown voltage of the TVB may be close to a withstanding voltage of the current balance circuit 145 and so the current balance circuit 145 may temporarily be pressed with a voltage higher than the withstanding voltage. The life spin of the current balance circuit 145 may be shortened. As a result, the circuit needing ESD protection is not equipped with the ESD protection circuit due to the reasons mentioned above and the circuit without ESD protection circuit has lower reliability and can not pass ESD association standards.